1. Field of the Invention
This invention relates in general to magnetic heads and in particular to magnetic heads of the flux-sensitive type.
2. Description Relative to the Prior Art
The playback of recorded signals from magnetic tape, or the like, using a conventional magnetic head that is sensitive to flux rate-of-change is difficult at low signal frequencies, and theoretically impossible for dc signals, or when there is no relative head-to-tape motion. Various techniques have been proposed for sensing tape flux, as opposed to the rate-of-change of flux (viz Hall effect devices; flux gate devices; etc.). The invention, as will be described below, employs a "single domain" thin magnetic film structure, say, one plated with permalloy to a thickness less than about three microns.
Thin film magnetometers have been described in the literature, and in a number of patents:
"Recent Advances in the Thin Film Inductance Variation Magnetometer", C. J. Bader and C. S. DeRenzi, Intermag, 1947, IEEE;
IEEE Transactions on Magnetics, Vol. Mag-8, #1, March 1972, "Magnetic Thin-film Magnetometers for Magnetic-Field Measurement", H. Irons and L. Schwee;
U.S. Pat. No. 2,856,581, issued in 1958 to L. Alldredge;
U.S. Pat. No. 3,012,177, issued in 1961 to H. Mortimer;
U.S. Pat. No. 3,239,754, issued in 1966 to W. Odom, Jr.; and
U.S. Pat. No. 3,271,665, issued in 1966 to P. Castro.
Of particular interest is the above-noted article by Bader and DeRenzi in which it is stated, categorically, that "variation (of film contributed inductance) with hard axis field (H.sub.y) . . . with no easy axis field, is zero for all values of H.sub.y)" (underlining added). Theoretically this may be the case, but it has been found to be incorrect for real films at various values of hard axis field. This fact is used to advantage in apparatus embodying the invention.